1. Technical Field
This invention relates to wireless communications systems and methods and wireless transmitter units for use therein and is especially concerned with radiated power and hence range of such wireless transmitter units in field operation.
2. Background Art
Radio frequency (RF) transmitters for control/communications/signaling systems are used in a wide variety of applications. In some applications, the transmitter is a hand-held, standalone unit, i.e., not part of a transceiver. One particular application is the monitoring of persons, such as hydro workers, forestry workers, oilfield workers, and so on, who work alone in remote or isolated locations outside of their vehicle. It is known for such workers to use a hand-held transmitter to communicate periodically with a receiver in the worker's vehicle, for example, to convey status or other signaling messages. The receiver in the vehicle is associated with a satellite transceiver whereby the status or signaling messages, along with position information, can be communicated to a central location or monitoring station via a satellite communications network.
It is generally desirable to have a longer operating range allowing the user to be farther from the receiver (vehicle) and still be able to use the transmitter to send an “OK” or “PANIC” signal back to the receiver. For systems employing unlicensed bands in the vicinity of 400 MHz, typical ranges achieved for safety systems vary from hundreds to thousands of feet.
Regulatory bodies, such as the FCC in the United States of America, have certain frequency bands allocated for unlicensed operation. These bands are popular choices for remote signaling systems because of the cost savings associated with unlicensed operation. To avoid spectral interference between systems, the regulations prescribe limits to RF radiation levels. In order to maximize range, RF radiation levels of the transmitters are set just below regulatory limits.
Since transmitted power cannot be increased beyond the prescribed limit, other approaches have been used to increase range. One approach is to increase receiver antenna gain. For vehicular applications requiring 360 degree azimuthal coverage, however, practical increases in receive antenna gain are limited.
Inadequate range may also result from the manner in which a particular transmitter is tested to determine compliance with the prescribed limit. More particularly, it is usual to operate the transmitter in a suitable chamber and measure its output using a field strength meter. In field operation, however, RF radiation levels are reduced by the presence of the operator's hand near the antenna. The hand absorbs RF radiation and potentially detunes the transmitter's antenna, thereby reducing the RF radiation level available at the receive antenna.
This effect might be mitigated to some extent by keeping the operator's hand as far away as possible from the antenna. For example, increasing the size of the transmitter housing, or providing a separate antenna, causes the controls and handgrip to be further away from the antenna. This approach would be undesirable, however, where a compact transmitter is required, as is usually the case.
Another approach might be to discourage handheld operation, by providing a wrist band, but this would be impractical or undesirable for most applications and, because the wrist would still be in close proximity to the antenna, performance would still be negatively affected.
To summarize, maximum radiation levels are limited by the regulations, which determine the maximum allowable RF radiation level for the transmitter in isolation. In field operation, the actual maximum radiation level is reduced by the proximity of the operator.